A radio coverage area in mobile communication systems is implemented with different cell sizes and often even with multilayer radio coverages of base stations. A cell having a very small coverage area, i.e. a microcell, is used for example in areas which are difficult to cover or require a high subscriber capacity. A larger coverage area is implemented with a macrocell, which often encompasses the geographical area of one or more microcells. In a multilayer radio coverage implemented in this way, macrocells represent coverage areas of a higher layer and microcells those of a lower layer. The mobile station may move from one cell to another and register with a new base station. When the mobile station has a call in progress, switching the call to the new cell is also involved, in which case switching to a new base station is referred to as handover.
FIG. 1 of the accompanying drawing discloses an example of a two-layer radio coverage in a mobile communication system. The simplified cell areas in FIG. 1 illustrate the areas where the base station of each cell can offer a sufficient signal level to the mobile station. In FIG. 1, a large macrocell M encompasses the areas of smaller microcells 1, 2 and 3. A mobile station MS located in position x within the microcell 1, for example, can in principle be served by the base station of either the macrocell M or the microcell 1.
In radio networks implemented with a multilayer radio coverage, it is advantageous to utilize the possibilities provided by the different cell layers by commanding the mobile stations, on the basis of some classification, to register with the most suitable base station, while also avoiding unnecessary handovers. One such classification principle is the speed of a mobile station in a mobile communication network having one or more microcells whose coverage areas are entirely inside another cell, typically a macrocell. It is known in the art that slow moving mobile stations are advantageously handed over to a base station of some microcell to prevent them from unnecessarily load the base station of the macrocell. On the other hand, it is advantageous to hand fast moving mobile stations over to the base station of the macrocell to prevent frequently recurring handovers and even to prevent calls from being lost when mobile stations move fast beyond the coverage area of the microcell.
FIG. 2 shows a simplified block diagram of the pan-European GSM mobile communication system. The units shown in FIG. 2 are present also in other mobile communication networks but they may have different names. A mobile station MS is connected over a radio path to one of base transceiver stations (BTS), such as BTS1 in FIG. 2. The mobile station MS constantly measures signals of the adjacent base stations for a possible handover. A base station system BSS comprises a base station controller BSC and base stations BTS under its control. Several base station controllers BSC generally operate under a mobile services switching centre MSC. The mobile services switching centre MSC is connected to other mobile services switching centres and via a gateway mobile services switching centre (GMSC) to a public switched telephone network. The operation of the whole system is supervised by an operation and maintenance centre OMC. Subscriber data of the mobile station MS is permanently stored in a home location register HLR and temporarily to a visitor location register VLR on whose area the mobile station MS is located at each time.
WO 95/28813 discloses a cellular mobile communication system where the time the mobile station is registered with a microcell is measured and the direction of travel of 0the mobile station is defined. A mobile station is indicated as being fast if the measured time in a cell is shorter than the pre-set time limit of this microcell and the entrance direction of the mobile station to the respective cell is different from the exit direction. In the prior art system, fast moving mobile stations are handed over to use a macrocell network. Handover from a microcell to a macrocell is carried out only if the mobile station has been found to be moving fast in several microcells. The slow speed of the handover decision is a problem with this solution. The mobile station has to pass through several microcells until a decision to carry out a handover to a macrocell can be made. During this time, the network is loaded with too frequent handovers from one microcell to another. A further problem is a possible blocking of a macrocell when all the mobile stations interpreted as being fast are handed over to the macrocell.
European Patent Application 0 589 278 discloses a microcell/macrocell system. This prior art system involves measuring the time a mobile station moving in a macrocell remains within a microcell inside the macrocell. The prior art system prevents fast moving mobile stations from being handed over to a base station of the microcell. Handover from the macrocell to the microcell is carried out only if the mobile station is still within the microcell after a predetermined time after its arrival in the microcell. If the mobile station has already departed from the microcell during the predetermined time, handover from the macrocell to the microcell will not be carried out. The slow speed of the handover decision is a problem with this solution, too. Handover decision can be made only after a predetermined time period after the mobile station has arrived in the microcell. After this time has passed, the mobile stations in the area of the microcell are interpreted as being slow and are handed over to a microcell although the mobile station would be just about to depart from the microcell. During the waiting time of the handover decision, the mobile station loads the macrocell. A problem with this solution is also an erroneous conception of the speed of a mobile station. The handover is affected only by the arrival of the mobile station in the microcell and by the location of the mobile station after a predetermined time from the arrival. During this time, a fast moving mobile station may have departed from the microcell and returned there, in which case the system carries out a handover from the macrocell to the microcell. In one embodiment of the system, the measurement of time is stopped if the mobile station departs from the microcell, and it is started again from the beginning when the mobile station returns to the microcell area. The timer will be reset, for example, if the mobile station moves beyond the coverage area of a microcell momentarily, or if the measurement result is erroneous/missing, in which case the mobile station will not be recognized as being slow.